Did you fill out only the withholding tax types in master data of the vendor or the withholding tax types and the withholding tax codes. If you have several withholding tax types. In that case remove the withholding tax codes and fill these out at the moment of the entering of the invoice.

In the vendor master, maintain the WHT tax type only. During invoice, the invoice processor will need to enter the relevant WHT code. The system will calculate WHT based on the WHT code that is entered.